The present invention is directed to processes for generating concealed images. More specifically, the present invention is directed to methods of placing concealed images on carbonless paper by ink jet printing processes. One embodiment of the present invention is directed to a process which comprises (1) incorporating into a printing apparatus capable of generating ink jet images a carbonless paper set comprising a first sheet, a second sheet, and optional intermediate sheets situated between the first sheet and second sheet, wherein the first sheet comprises paper coated on one surface with a color former and the second sheet comprises paper coated on one surface with a color developer, and wherein, when the carbonless paper set is assembled, the surface of the first sheet coated with the color former is in contact with the surface of a sheet coated with the color developer and the surface of the second sheet coated with the color developer is in contact with the surface of a sheet coated with the color former; (2) incorporating into the printing apparatus an ink jet ink comprising (a) water; (b) a member selected from the group consisting of glycols, sulfolane, and mixtures thereof; (c) optionally, a member selected from the group consisting of ethanol, isopropanol, n-butanol, benzyl alcohol, hexanetriol, 1,6-hexanediol, 1,5-pentanediol, 1,4-butanediol, 1,3-butanediol, and mixtures thereof; (d) diethylene glycol monobutyl ether; (e) optionally, dipropylene glycol monomethyl ether acetate; and (f) a marking material which is substantially colorless and detectable when exposed to radiation outside of the visible wavelength range; and (3) causing droplets of the ink to be ejected in an imagewise pattern onto at least one surface of at least one of the sheets of the carbonless paper set.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. No. 4,601,777, U.S. Pat. No. 4,251,824, U.S. Pat. No. 4,410,899, U.S. Pat. No. 4,412,224, and U.S. Pat. No. 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Carbonless paper sets generally are stacks of at least two sheets of paper wherein the application of pressure in imagewise fashion on the top sheet, typically by handwriting or typing, results in formation of a corresponding image on the underlying sheets, so that copies are formed as the image is generated on the top sheet. Carbonless paper sets typically comprise a top sheet of paper, on the bottom surface of which is coated a first composition, and a bottom sheet, on the top surface of which is coated a second composition. The first and second compositions are in contact with each other when the top and bottom sheets are placed in stack formation, and generally are of a nature such that application of pressure to the top sheet of the stack at a specified location causes interaction between the first and second compositions that results in the formation of a colored area on the bottom sheet at the location at which pressure was applied. Intermediate sheets can be located between the top and bottom sheets, wherein each intermediate sheet is coated on its top surface with the second composition and on its bottom surface with the first composition; application of pressure to the top sheet then results in the formation of a colored area at the location at which pressure was applied on each of the intermediate sheets and on the bottom sheet.
An example of a carbonless paper set is disclosed in U.S. Pat. No. 3,843,383, the disclosure of which is totally incorporated herein by reference. This patent discloses a recording sheet comprising a support having thereon a layer of color developer capable of reacting with a substantially colorless color former to form colored images. The paper set generally comprises a top sheet coated with microcapsules containing a color former solution, a bottom sheet coated with a color developer material in a binder, and, in some instances, middle sheets coated on one side with the color developer and on the other side with the color former microcapsules. Alternatively, the color former microcapsules and the color developer can be applied to the same surface of a paper. The color developer comprises a clay into which is incorporated at least one aromatic carboxylic acid or alkali metal salt thereof, and, optionally, acidic resins or inorganic pigments such as metal oxides, metal hydroxides, or metal carbonates. Suitable clays include acidic clay, active clay, attapulgite, zeolite, bentonite, kaolin, silicic acid, synthetic silicic acid, aluminum silicate, zinc silicate, colloidal silicic acid, and the like. The clay and the aromatic carboxylic acid or alkali metal salt thereof are formed into a coating solution which is then applied to paper. The color former is dissolved in a solvent and encapsulated in microcapsules, or is dissolved in a solvent and mixed with a binder. Contacting a sheet coated with microcapsules containing the color former under pressure with a sheet coated with the color developer results in formation of a color image. Other patents disclosing carbonless paper of this type include U.S. Pat. No. 2,712,507 and U.S. Pat. No. 2,730,456, the disclosures of which are totally incorporated herein by reference. Alternatively, as disclosed in U.S. Pat. No. 2,730,457, the disclosure of which is totally incorporated herein by reference, the color former microcapsules and the color developer of a carbonless paper can be applied to the same surface of a paper sheet. Other configurations of color former, color developer, and a pressure-releasable liquid solvent are possible, including, for example, those disclosed in U.S. Pat. No. 3,672,935, the disclosure of which is totally incorporated herein by reference. Additional patents disclosing carbonless papers and materials suitable for carbonless paper applications include U.S. Pat. No. 2,417,897, U.S. Pat. No. 3,672,935, U.S. Pat. No. 3,681,390, U.S. Pat. No. 4,202,820, U.S. Pat. No. 4,675,706, U.S. Pat. No. 3,481,759, U.S. Pat. No. 4,334,015, U.S. Pat. No. 4,372,582, U.S. Pat. No. 4,334,015, U.S. Pat. No. 2,800,457, U.S. Pat. No. 2,800,458, U.S. Pat. No. 3,418,250, U.S. Pat. No. 3,516,941, U.S. Pat. No. 4,630,079, U.S. Pat. No. 3,244,550, U.S. Pat. No. 3,672,935, U.S. Pat. No. 3,732,120, U.S. Pat. No. 3,843,383, U.S. Pat. No. 3,934,070, U.S. Pat. No. 3,481,759, U.S. Pat. No. 3,809,668, U.S. Pat. No. 4,877,767, U.S. Pat. No. 4,857,406, U.S. Pat. No. 4,853,364, U.S. Pat. No. 4,842,981, U.S. Pat. No. 4,842,976, U.S. Pat. No. 4,788,125, U.S. Pat. No. 4,772,532, and U.S. Pat. No. 4,710,570, the disclosures of each of which are totally incorporated herein by reference.
Frequently carbonless paper sets are printed as forms, wherein a large number of sets are printed with standard text or other material, leaving blank areas for individualized information to be filled in by, for example, impact typewriting or handwriting. Typically, carbonless preprinted forms are generated by techniques such as offset printing. Offset printing and other large scale printing processes, however, require complex and expensive equipment which is not generally found in an office or small business environment. Thus, one desiring forms printed on carbonless paper generally must order them from a professional printer, thus generating added costs and inconvenience, particularly when only a relatively small number of the pre-printed forms are needed. The ability to generate pre-printed carbonless forms on standard office equipment thus can be desirable, particularly when small quantities of forms are desired.
In some instances, it may be desirable to print concealed images on carbonless forms, such as barcodes. Concealed images, for the purposes of the present invention, are images that are invisible to the human eye under normal viewing conditions, but readable by a sensor, such as an infrared-sensitive device or a fluorescence detector, or by the human eye when the images are exposed to radiation outside of the visible wavelength range, such as ultraviolet light or the like. For example, the pharmaceuticals industry uses large numbers of carbonless forms for clinical drug testing; after each test has been completed and each form filled in, the completed forms can be returned to the pharmaceutical company for collation and analysis of the data. Invisible images can be useful in this context to tag each copy for subsequent processing. Many other possible applications exist for concealed images on carbonless forms. The marking material employed to generate the concealed images, however, must be of a composition such that it does not adversely interact with the carbonless paper color former or color developer materials; such interaction would impair the ability to generate carbonless images by application of pressure to the carbonless set subsequent to printing of the concealed image.
U.S. Pat. No. 5,174,556 (Taylor et al.), the disclosure of which is totally incorporated herein by reference, discloses a document finisher which includes a printing station for printing on the binding of a book. The printing station in one embodiment prints on the binder tape before the book is bound. In a second embodiment, the printer prints on the binding after the book is bound. The printing stations are space efficient and designed to be easily incorporated with preexisting stations in document finishers. Ink jet printers and impact-type printer may be utilized.
U.S. Pat. No. 5,156,675 (Breton et al.), the disclosure of which is totally incorporated herein by reference, discloses fast drying ink compositions containing a colorant, a dye, water and a cosolvent. Some of the ink compositions dry in less than about I second and have a viscosity of between about 1.6 and about 2.5 centipoise and a specified surface tension. Some of the ink compositions contain specified cosolvents, preferably a mixture of diethylene glycol monobutyl ether and glycerol.
European Patent Publication 518,490, published Dec. 16, 1992, equivalent of copending application U.S. Ser. No. 07/700,967, filed May 16, 1991, entitled "Ink Jet Compositions Containing Desizing Agents," with the named inventors Marcel P. Breton, Shadi L. Malhotra, Toshitake Yui, Kerstin M. Henseleit, and Melvin D. Croucher, the disclosure of which is totally incorporated herein by reference, discloses an ink composition having a pH of about 5 to about 10 containing a colorant, a liquid carrier, and less than about 5 wt. % of a desizing agent. The desizing agent may be (1) poly(oxyalkylene) modified compounds of sorbitan esters, fatty amines, alkanol amides, castor oil, fatty acid, fatty alcohol; (2) hydrophilic poly(dialkyl-siloxanes); (3) fatty imidazolines; (4) fatty ester modified compounds of phosphate, sorbitan, glycerol, poly(ethylene glycol), sulfosuccinic acid, sulfonic acid, alkyl amine; (5) quaternary alkosulfate compounds; (6) poly(propylene oxide) - poly(ethylene oxide) copolymers; (7) poly( alkylene glycol); or (8) mixtures thereof.
Copending application U.S. Ser. No. 07/616,971, filed Nov. 21, 1990, entitled "Carbonless Paper for Ink Jet Printing," with the named inventors John F. Oliver, Richard E. Sandborn, and David J. Sanders, the disclosure of which is totally incorporated herein by reference, discloses a process for generating images which comprises incorporating into an ink jet printing apparatus a carbonless paper set which comprises a first sheet comprising a support containing a color developer capable of reacting with a color former to produce a color image, said color developer comprising high surface area silica particles, and a second sheet comprising a support coated with the color former, forming an image on the first sheet by causing ink to be expelled in droplets on a surface containing the color developer, and forming an image on the second sheet by causing ink to be expelled in droplets onto the surface opposite to that coated with the color former.
Copending application U.S. Ser. No. 07/806,508, filed Dec. 13, 1991, entitled "Carbonless Paper for Electrostatic Imaging Processes," with the named inventors David J. Sanders, John F. Oliver, and Marcel P. Breton, the disclosure of which is totally incorporated herein by reference, discloses a process which comprises incorporating into an electrostatic imaging apparatus a recording sheet comprising a support on one surface of which are situated microcapsules which comprise a shell and a core containing a color former and an oil, said microcapsules being strengthened with a polymer capable of degrading upon exposure to actinic radiation; generating an electrostatic latent image on an imaging member in the apparatus; developing the latent image with a developer; transferring the developed image to the recording sheet; and, subsequent to transfer, exposing the recording sheet to actinic radiation at a wavelength at which the polymer will degrade, thereby rendering the microcapsules subject to rupture upon application of pressure.
Copending application U.S. Ser. No. 07/922,882, filed Jul. 31, 1992, entitled "Colorless Fast-Drying Ink Compositions for Printing Concealed Images Detectable by Fluorescence," with the named inventors Fran.cedilla.oise M. Winnik, Anthony R. Davidson, and Marcel P. Breton, the disclosure of which is totally incorporated herein by reference, discloses an ink composition consisting essentially of water, diethylene glycolmonobutyl ether (butyl carbitol), glycerol, an optional cyclohexyl pyrrolidinone component, a dye selected from the group consisting of dyes containing dansyl chromophores and dyes containing porphyrin chromophores, an optional biocide, and an optional polyalkylene oxide/bisphenol-A additive.
Copending application U.S. Ser. No. 07/877,502, filed May 1, 1992, entitled "Xerographic/Thermal Ink Jet Combined Printing," with the named inventors Thomas N. Taylor, LeRoy A. Baldwin, and Otto R. Dole, the disclosure of which is totally incorporated herein by reference, discloses a printer which combines the technologies of xerographic and thermal ink jet printing into a unit capable of high resolution text and color graphics. The printer is capable of forming a composite image, including a xerographic printing portion and a thermal ink jet (TIJ) printing portion, by printing the xerographic portion using known xerographic techniques and the thermal ink jet portion by a thermal ink jet printing array associated with the printer. The portions may be printed in any order, and may be dried by a drying station after printing of each portion or after both portions have been printed. At least one thermal ink jet printing array can serve as an annotator which is capable of printing additional information onto a copy, such as company letterhead, special instructions, addresses, or the like.
Accordingly, while known materials and processes are suitable for their intended purposes, a need remains for processes for printing concealed images on carbonless papers. In addition, a need remains for processes for printing concealed images on carbonless papers by methods entailing the use of standard office equipment such as ink jet printers. Further, there is a need for processes for printing concealed images on carbonless papers wherein the ink employed does not interact with the color former or color developer components of the carbonless paper in such a manner as to impair the pressure developability of the paper. Additionally, there is a need for processes for printing fast-drying concealed images on carbonless papers by ink jet printing methods without impairing the pressure developability of the paper. There is also a need for processes for printing concealed images on carbonless papers by ink jet printing methods wherein the resulting images are of good quality and contrast with respect to the background areas of the paper when viewed under the desired viewing conditions (such as exposure to ultraviolet light, infrared light, or the like). Further, there is a need for processes for printing concealed images on carbonless papers by ink jet printing methods wherein the carbonless papers also contain visible images generated by another printing method, such as electrophotography, electrography, ionography, lithography, offset printing, or the like. In addition, a need remains for processes for printing concealed images on carbonless papers by ink jet printing methods wherein the ink jet inks exhibit rapid drying times on the carbonless paper, thereby enabling the annotation via ink jet printing of xerographic copies with markings not visible to the naked eye in rapid document generation processes with no smearing of the ink jet images.